A transfer function or transmission response represented as a function of frequency characterizes a performance of a radio frequency (RF) device under test (DUT) in a frequency domain. The transmission response, sometimes also termed a ‘complex transmission response’, is generally specified either in terms a real response and an imaginary response (i.e., as a complex number) or equivalently in terms of a magnitude response and a phase response of the DUT as a function of frequency across a band or range of frequencies of interest. For example, the transmission response of a DUT may comprise a magnitude response and a phase response of the DUT measured or otherwise determined at a plurality of frequencies spanning an operational frequency range of the DUT. The transmission response essentially characterizes an effect that the DUT has on signals passing through the DUT.
The transmission response is normally measured by introducing a test signal at a port or ports of the DUT and measuring the signal as it exits at another port or ports of the DUT. A magnitude and a phase of the test signal entering and exiting the DUT are compared. A measurement or determination of the transmission response of the DUT is derived from the comparison.
For example, a transmitter and a receiver of a measurement system may be attached to a first port (i.e., port-1) and a second port (i.e., port-2) of the DUT, respectively. By comparing the magnitude and the phase of the signal received by the receiver with the magnitude and the phase of the signal transmitted by the transmitter, the magnitude response and the phase response of the DUT are measured. A vector network analyzer (VNA) that measures a magnitude and a phase of a forward transmission S-parameter (e.g., S21) using a swept RF signal is but one example of a measurement system that is employed to determine such a complex transmission response of a DUT.
Unfortunately, in some situations one of the first port and the second port of the exemplary DUT may not be readily accessible for connection to the measurement system (e.g., VNA). For example, when attempting to determine a transmission response of a DUT comprising a long cable installed in a cellular telephone tower or in a wing of an aircraft, one or more of a distance between a first end and a second end of the cable and an inability to readily or directly access one of the ends may preclude connecting a VNA simultaneously to both ends of the cable. In another example, a second end of a DUT may be connected to another device, such as an antenna, in a manner that does not facilitate disconnection and subsequent attachment to a VNA. In such exemplary situations, a conventional forward S-parameter measurement of the DUT using a VNA to determine the transmission response may not be possible or, at the very least, is rendered inconvenient.